[libc++][TZDB] Fixes reverse time lookups. (#89502)

Testing with the get_info() returning a local_info revealed some issues
in the reverse lookup. This needed an additional quirk. Also the
skipping when not in the current continuation optimization was wrong. It
prevented merging two sys_info objects.

Author: mordante

libcxx/src/time_zone.cpp

@@ -567,11 +567,22 @@ __first_rule(seconds __stdoff, const vector<__tz::__rule>& __rules) {
         false};
   }
 
-  __named_rule_until __continuation_end{__continuation};
-  if (__time >= __continuation_end.__until() && !__continuation_end.__needs_adjustment())
-    // note std::unexpected<sys_seconds>(__end); is ambiguous with std::unexpected() in <exception>,
-    return __sys_info_result{std::unexpect, __continuation_end.__until()};
+  if (__rule->__save.__time != 0s) {
+    // another fix for America/Punta_Arenas when not at the start of the
+    // sys_info object.
+    seconds __save = __rule->__save.__time;
+    if (__continuation_begin >= __rule_begin - __save && __time < __next.first) {
+      return __sys_info{
+          sys_info{__continuation_begin,
+                   __next.first,
+                   __continuation.__stdoff + __save,
+                   chrono::duration_cast<minutes>(__save),
+                   chrono::__format(__continuation, __rule->__letters, __save)},
+          false};
+    }
+  }
 
+  __named_rule_until __continuation_end{__continuation};
   while (__next.second != __rules.end()) {
 #ifdef PRINT
     std::print(

libcxx/test/std/time/time.zone/time.zone.timezone/time.zone.members/get_info.sys_time.pass.cpp

@@ -1299,6 +1299,78 @@ static void test_america_indiana_knox() {
                tz->get_info(to_sys_seconds(2006y, std::chrono::October, 29d, 6h, 59min, 59s)));
 }
 
+static void test_america_punta_arenas() {
+  // Z America/Punta_Arenas -4:43:40 - LMT 1890
+  // ...
+  // -4 - -04 1919 Jul
+  // -4:42:45 - SMT 1927 S
+  // -5 x -05/-04 1932 S
+  // ...
+  //
+  // R x 1927 1931 - S 1 0 1 -
+  // R x 1928 1932 - Ap 1 0 0 -
+  // ...
+
+  using namespace std::literals::chrono_literals;
+  const std::chrono::time_zone* tz = std::chrono::locate_zone("America/Punta_Arenas");
+
+  assert_equal(
+      std::chrono::sys_info(
+          to_sys_seconds(1927y, std::chrono::September, 1d, 4h, 42min, 45s),
+          to_sys_seconds(1928y, std::chrono::April, 1d, 4h),
+          -4h,
+          60min,
+          "-04"),
+      tz->get_info(to_sys_seconds(1927y, std::chrono::September, 1d, 4h, 42min, 45s)));
+
+  assert_equal(
+      std::chrono::sys_info(
+          to_sys_seconds(1927y, std::chrono::September, 1d, 4h, 42min, 45s),
+          to_sys_seconds(1928y, std::chrono::April, 1d, 4h),
+          -4h,
+          60min,
+          "-04"),
+      tz->get_info(to_sys_seconds(1928y, std::chrono::April, 1d, 3h, 59min, 59s)));
+}
+
+static void test_europ_ljubljana() {
+  // Z Europe/Ljubljana 0:58:4 - LMT 1884
+  // 1 - CET 1941 Ap 18 23
+  // 1 c CE%sT 1945 May 8 2s
+  // 1 1 CEST 1945 S 16 2s
+  // 1 - CET 1982 N 27
+  // 1 E CE%sT
+  //
+  // ...
+  // R c 1943 o - O 4 2s 0 -
+  // R c 1944 1945 - Ap M>=1 2s 1 S
+  // R c 1944 o - O 2 2s 0 -
+  // R c 1945 o - S 16 2s 0 -
+  // R c 1977 1980 - Ap Su>=1 2s 1 S
+  // ...
+
+  using namespace std::literals::chrono_literals;
+  const std::chrono::time_zone* tz = std::chrono::locate_zone("Europe/Ljubljana");
+
+  assert_equal(
+      std::chrono::sys_info(
+          to_sys_seconds(1945y, std::chrono::April, 2d, 1h),
+          to_sys_seconds(1945y, std::chrono::September, 16d, 1h),
+          2h,
+          60min,
+          "CEST"),
+      tz->get_info(to_sys_seconds(1945y, std::chrono::April, 2d, 1h)));
+
+  assert_equal(
+      std::chrono::sys_info(
+          to_sys_seconds(1945y, std::chrono::April, 2d, 1h),
+          to_sys_seconds(1945y, std::chrono::September, 16d, 1h),
+          2h,
+          60min,
+          "CEST"),
+      tz->get_info(to_sys_seconds(1945y, std::chrono::September, 16d, 0h, 59min, 59s)));
+}
+
 int main(int, const char**) {
   // Basic tests
   test_gmt();
@@ -1333,5 +1405,9 @@ int main(int, const char**) {
   test_america_ciudad_juarez();
   test_america_indiana_knox();
 
+  // Reverse search bugs
+  test_america_punta_arenas();
+  test_europ_ljubljana();
+
   return 0;
 }